A charge amplifier by which electric charge generated in a piezoelectric pressure sensor grounded to a mount target is converted into a voltage signal has been heretofore formed by use of an operational amplifier. Combustion pressure of an internal combustion engine, however, varies to both a positive value and a negative value. Therefore, there is employed a method in which the operational amplifier is operated by a double power source (having a pair of positive and negative power sources, for example, of ±5 V) to convert electric charge into a voltage signal.
For example, a charge amplifier shown in FIG. 17 is one of this type charge amplifiers. In the charge amplifier 501, an inverted input terminal of an operational amplifier 505 is connected, through a resistor 503, to the other end of a piezoelectric pressure sensor S having one end grounded to a mount target. The inverted input terminal is connected to an output terminal through a feedback capacitor 507. A feedback resistor 509 is further connected in parallel to the feedback capacitor 507. On the other hand, a non-inverted input terminal is grounded. A power source of +5 V is connected to a positive power source input terminal of the operational amplifier 505 whereas a power source of −5V is connected to a negative power source input terminal of the operational amplifier 505. The operational amplifier 505 is driven by the two power sources.
It is however general that electric power is supplied from a battery of a plus power source when combustion pressure of an internal combustion engine of an automobile needs to be measured. To use the operational amplifier by the two power sources in such a case, it is necessary to form a circuit of a minus power source from a DC-DC converter, etc. This causes increase in cost.